The Physical Medium Dependent (PMD) sublayer is defined in IEEE 802.3 clause 38 for 1000BASE-LX and 1000BASE-SX (long and short wave laser). This type of PMD sublayer is provided by the external gigabit interface converter (GBIC) or small form-factor pluggable (SFP) optical transceiver, which should be connected directly to the ports of the GT transceiver.